Clearing the Smoke on Cannabis

Effects on Neurocognitive Functioning
Maternal cannabis use during pregnancy has subtle effects on offspring’s neurocognitive functioning.

Beginning at age three to four, children of mothers who used cannabis heavily while pregnant have demonstrated deficits in memory, verbal and perceptual skills, and verbal and visual reasoning after adjusting for potentially confounding variables (Day et al., 1994; Fried & Watkinson, 1990).

Impaired performance in verbal and quantitative reasoning and short-term memory has also been found among six-year-old children whose mothers reported smoking one or more marijuana cigarettes per day, after controlling for significant covariates (Goldschmidt, Richardson, Willford, & Day, 2008).

Vulnerability in visual-cognitive functioning has been shown to persist into early adolescence among those offspring heavily exposed to cannabis (Fried, Watkinson, & Gray, 2003). Findings from brain imaging studies of young adults aged 18–22 indicate that in utero cannabis exposure negatively impacts the neural circuitry involved in aspects of executive functioning, including response inhibition and visuospatial working memory (Smith, Fried, Hogan, & Cameron, 2004, 2006).

These findings are particularly noteworthy as they demonstrate the long-term impairing effects of prenatal exposure to cannabis on offspring’s neurocognitive functioning.Global intelligence does not appear to be impacted by prenatal cannabis exposure (Fried et al., 1998, 2003).

When children reach age six, the effects of maternal cannabis use during pregnancy become much more evident. Compared to offspring of non-users, children born to cannabis users— particularly heavy users—have been found to be more hyperactive, inattentive, and impulsive (Fried, Watkinson, & Gray, 1992; Leech, Richardson, Goldschmidt, & Day, 1999), even after controlling for extraneous variables.

At age 10, prenatally exposed children display increased hyperactivity, inattention, and impulsivity, and show increased rates of delinquency and externalizing problems as reported by their mothers and teachers, compared to those children who were not exposed prenatally to cannabis (Fried et al., 1998; Goldschmidt, Day, & Richardson, 2000).

In children aged 13–16, however, some aspects of attention (i.e., flexibility, encoding and focusing) appear to no longer be affected by cannabis exposure (Fried et al., 2003).

There is accumulating evidence that suggests prenatal cannabis exposure may contribute to the initiation and frequency of subsequent substance use during
adolescence. Porath and Fried (2005) reported that 16- to 21-year-old offspring (particularly males) of cannabis users were at increased risk, in a dose-related
manner, for the initiation of cigarette smoking and cannabis use, and daily cigarette smoking, compared to offspring of non-using mothers, independent of potential prenatal confounds. Similar results were noted by Day, Goldschmidt, and Thomas (2006); compared to offspring of non-users, youth of mothers who heavily used cannabis while pregnant not only reported using this substance more frequently at age 14, but they also initiated use at an earlier age. This result was significant even after controlling for potential confounds.

Effects on Mental Health
There is emerging evidence linking in utero cannabis exposure to depressive and anxious symptomatology. After controlling for prenatal exposure to other drugs and risk factors for childhood depression, offspring of maternal cannabis users expressed significantly more depressive and anxious symptoms at age 10 compared to children of non-users (Gray, Day, Leech, & Richardson, 2005; Leech, Larkby, Day, & Day, 2006).

Mechanisms of Action
The mechanisms responsible for the effects of prenatal cannabis exposure are not well understood. Cannabinoids are able to cross the placental barrier and may affect the expression of key genes for neural development, leading to neurotransmitter and behavioural disturbances (Gomez et al., 2003). The presence of cannabinoid receptors in the placenta and fetal brain may also mediate adverse actions of prenatal cannabis exposure (Park, Gibbons, Mitchell, & Glass, 2003), as these receptors are associated with aspects of brain functioning including cognition and memory (Kumar, Chambers, Pertwee, 2001). Animal studies have documented that cannabinoids can lead to changes in dopamine activity and impaired functioning of the hypothalamus-pituitaryadrenal axis (Kumar et al., 2001), which may affect mood and neurobehavioural outcomes in offspring. It is also possible that an underlying genetic factor may account for both the lifestyle habits of the pregnant mother (i.e., cannabis use) and her child’s neurodevelopment and behaviour.

Conclusions and Implications
Evidence does suggest that prenatal exposure to cannabis (particularly heavy exposure) has subtle adverse effects, beginning at approximately three years of age, on subsequent cognitive functioning, behaviour, substance use, and mental health in offspring. Cannabis-related deficits in the cognitive domain could impair a child’s academic functioning and may require educational remediation, enrichment or placement to help protect against future learning problems.

Prevention efforts directed towards reducing maternal cannabis use during pregnancy could have significant effects in reducing such cognitive impairment. Prevention and intervention programs aimed at reducing prenatal cannabis exposure could also help reduce the percentage of youth who experience mental health conditions and other comorbid problem behaviours, such as substance use and delinquency. It has been reported that at least half of all pregnancies in North America are unplanned (Walker, Rosenberg, & Balaban, 1999). That, combined with the fact that nearly 7% of American women of childbearing age (15–44 years) reported past-month use of marijuana and hashish in 2005 (SAMHSA, 2006) indicates the potential risk for offspring to be prenatally exposed to cannabis.

Cannabis use is a preventable prenatal risk factor; the findings reviewed from the literature suggest that it is prudent to advise pregnant women, and women thinking of becoming pregnant, of the risks associated with cannabis use during pregnancy.